1. Field of the Invention
The present invention relates generally to the field of blood pressure monitoring, and more particularly to automatic monitoring of systolic blood pressure.
2. Description of the Prior Art
The prior art is replete with devices for measuring systolic pressure of a living subject. An old and simple device is a pressurizable cuff used in combination with a mercury manometer which reads pressure in the cuff and a stethoscope which is used to listen to Korotkoff sounds. More complicated methods and apparatus based on the same principle of listening to the Korotkoff sounds replace the mercury manometer with a mechanical or electromechanical pressure gauge and utilize microphonic detection of the Korotkoff sounds which are analyzed electrically. In another advanced method of measuring blood pressure, the distance from a blood pressure cuff to the wall of an artery is accurately determined by measuring Doppler shifts of sound waves reflected by the artery. The distance to the artery varies as a function of pressure within the somewhat pliable walls thereof. In yet other methods for measuring blood pressure intrusive devices are often inserted directly into blood vessels.
Oscillometric methods of determining systolic pressure are also well known in the art. In such methods, the operator observes a representation of the strength of pulsations of pressure within an artery. This can be done visually, as by watching the extent of bouncing of the top of a mercury column in a mercury manometer which is in pressure-communication with a cuff, or indirectly as by measuring the occlusion which occurs to a blood vessel in the pinna of the ear as pressure is exerted thereon, as in U.S. Pat. No. 3,227,155. Oscillometric methods of determining systolic pressure generally define systolic pressure to be the maximum applied pressure at which threshold oscillations are observed to occur. With a typical mercury manometer and pressurizable cuff, this pressure would then be the highest pressure at which the operator noted bouncing in the top of the mercury column as the pressure in the cuff was slowly and relatively uniformly reduced. However, there are inaccuracies associated with this method for determining threshold oscillations, since the mercury column does not noticably respond to narrow-width pressure pulses; i.e., the energy associated with a narrow-width pulse is insufficient to noticably move the comparatively high inertia mercury column. In other words, because of relatively slow response time of a mercury manometer (or the apparently selectively slow occlusion rate of the pinna of the ear) the quantity actually being measured is proportional to an integral of the pressure pulse rather than actual amplitude thereof. Oscillometric methods based on observing threshold oscillations are thus inherently somewhat inaccurate, where "threshold" is a parameter or term that generally may be hard to rigorously and exactly define anyway.
Nevertheless, methods based on listening to Korotkoff sounds are relatively accurate for measuring systolic pressure, but are burdened with requiring use of a microphonic detector if they are to be instrumented. The method based on Doppler shifts is also accurate, but also is burdened with requiring special measuring apparatus, and has a further shortcoming in that it is sensitive to positioning of the measuring apparatus relative to the artery.
The present invention provides a solution to the problems associated with inaccurate systolic blood pressure measurement and monitoring provided by simple devices of the prior art. The present invention also provides a solution to the problems associated with complex and special microphonic and other apparatus employed in the more accurate prior art devices for measuring and monitoring systolic blood pressure. The present invention thus provides apparatus and method for automatically measuring and monitoring systolic blood pressure, employing a simple cuff and automatically controlled instrumentation.
A related U.S. patent application Ser. No. 445,559 filed Feb. 25, 1974 and now U.S. Pat. No. 3,903,872, issued Sept. 9, 1975, and assigned to the assignee of the present invention, entitled "Apparatus and Method for Producing Sphygmometric Information," is directed to diastolic blood pressure, and is incorporated herein by reference.